Method of manufacturing free machining stainless steel

ABSTRACT

DESCRIBED HEREIN IS AN IMPROVED METHOD OF MANUFACTURING FREE MACHINING STAINLESS STEELS TO AVOID UNDULY ELONGATING THE INCLUSIONS CONTAINED THEREIN.

United States Patent ABSTRACT OF THE DISCLOSURE 3 Claims Described herein is an improved method of manufacturing free machining stainless steels to avoid unduly elongating the inclusions contained therein.

This invention relates to a method of treating free machining steels to improve machinability. More particularly, the invention involves a method of treating stainless steels exhibiting a gamma loop type of phase relationship with temperature, e.g., sulfide-bearing steels, to produce a desirable metallurgical structure typical of good machining steels wherein the sulfides are present as relatively short, thick particles. The method in accordance with the invention involves performing substantially all hot rolling of the steel in the delta ferrite temperature range.

There has been shown to be a wide variation in the machinability of free machining steels. This is also true of bars which may come from the same heat or ingot of material. Other factors being equal the differences are normally related to the structure of the inclusions. Such factors as cooling rate in the ingot, tapping temperature, ingot size, position in the ingot and steel composition influence the size and distribution of the as-cast inclusions. It has been found, however, that where process conditions are carefully controlled and kept relatively constant, there still appears a considerable variation in the inclusion size in cooled bars and billets. By practicing the present invention it is possible to control the inclusion size and shape in finished bars of free machining stainless steels to produce more uniform and higher average levels of machinability.

In accordance with the invention there is provided a method of treating free machining stainless steels of the type which exhibit a gamma loop type of phase relationship with temperature and which contain sulfur and similar elements, e.g. selenium and tellurium. Typical of this category of steels is a stainless steel known as Type 416. This steel consists essentially of up to 0.15% carbon, up to 1.25% manganese, up to 0.06% phosphorus, at least 0.15% sulfur, up to 1.0% silicon, up to 0.6% molybdenum, 12 to 14% chromium and the balance essentially iron. It is believed that the rates at which the sulfide inclusions are reduced in cross sectional area relative to the matrix metal are related to the microstructure of the metal at hot rolling temperature. It has been learned that as the temperature is raised into the delta ferrite temperature range increasing amounts of delta ferrite are formed and this phase structure is characteristically soft and plastic. In this range the metal appears to deform more readily than the sulfides. Below the delta ferrite temperature range the structure is essentially austenite which appears to be relatively stronger and more difficult to deform than the sulfides. Consequently, in the higher temperature range the inclusions are not reduced in area as readily as the metal matrix and tend to remain thick and short. In contrast at temperatures below the delta ferrite range the sulfide inclusions are reduced in diameter and elongate at approximately the same rate as the matrix and, therefore, tend to become long and thin.

The following examples will illustrate the practice of one embodiment of the invention. Sections from produc- 3,598,660 Patented Aug. 10, 1971 tion billets of Type 416 stainless steel having the composition described in Table I which had 'been rolled by conventional practices from the ingot were hot rolled in three temperature ranges to finished bars. The ranges were 2300-2150 F., 2050-1900 F. and 1850-1700 F., starting and finishing temperatures respectively. The rolling sequence was to 1%" square, 1" diameter, diameter and /2 diameter. In order to maintain the desired temperature range the bars were periodically reheated between mill passes. The average sulfide inclusion diameter, determined at the billet stage, %1" and /z" diameter bar stages for bars from three billets is given in Table II. The ratio between the actual inclusion diameter and a calculated diameter assuming the same reduction rate as the bar was calculated. A ratio value of one would indicate that the sulfide were reduced at the same rate as the bar. The higher the ratio, the slower the rate at which inclusion diameter was reduced relative to the bar diameter. The results of the foregoing are described in Table II.

TABLE I Heat No. 0 Mn S Si 1 Balance essentially iron.

TABLE II Ratio of relative Median inclusion Rolling inclusion diameter to Heat and temperature, diameter relative bar billet N 0. Size range (microns) diameter 93128-T1 4 x 4 billet 5.0 round.-. 2, 300-2, 150 1. 60 1. 94 do. 1, 850-1, 700 0.80 1. 03 round- 2, 300-2, 150 1. 50 2. 72 .--do. 2, 050-1, 900 0. 1. 55 do 1, 850-1, 700 0. 70 1. 27

93l28-B7 4 x 4 billet 5. 7 round- 2, 300-2, 150 2. 05 2. 18 -do 1, 850-1, 700 1. 05 1. 11 round..- 2, 300-2, 150 1. 60 2. 56 do 2, 050-1, 900 1. 05 1. 67 do 1, 850-1, 700 0. 80 l. 27

billet. round-.. 2, 300-2, 150 2. 35 2. 46 1, 850-1, 700 0. 0. 94 2, 300-2, 1. 70 2. 66 2, 050-1, 900 1. 25 1. 97 1, 850-1, 700 0. 80 1. 25

In each of the examples above the average diameters of the inclusions in the bars rolled at 2300-2150" F. in at least twice the diameters as those in bars rolled 1850- 1700 F. Also in the /2" bars the difference between the inclusion sizes in bars rolled at the intermediate temperature (2050-1900 F.) and the highest range (2300- 2150 F.) are proportionately much greater than they are between the intermediate and lowest (18501700 F.) range. Moreover, in the 2300-2150 F. range the reduction rate of the inclusions in rolling from the billet to /2" diameter bars is between /2 and /3 the rate at which the bar is reduced. In the 1850-1700 F. range the rate approaches the same rate as the bar reduction rate.

The effects described above occur as they do because the delta ferrite region begins about 2000 Thus, Type 416 should in accordance with the invention, be hot rolled at temperatures above 2000 F., preferably higher to develop the most satisfactory inclusion structure. In general the ferrite present in the microstructure is a function of the carbon content in temperature for medium or high chromium steels. In commercial practice, after hot rolling, the steel is air cooled, usually to room temperature, and heat treated. Heat treatment may comprise subcritical annealing at 1000 to 1550? for up to 4 hours or hardening and tempering by heating at 1750 to 1900 F., rapidly cooling and tempering at temperatures up to 1000 F.

.I claim:

1. An improvement in the manufacture of free machining stainless steels which exhibit a gamma loop type of phase relationship with temperature to provide inclusions as short, thick particles of minimum elongation which comprises performing substantially all hot rolling of said steel in the delta ferrite temperature region to reduce section thickness so that the elongation of 'said inclusions is disproportionately less than the reduction in cross sectional area and elongation of the steel as a whole.

2. A method according to claim 1 wherein said steel consists essentially of up to 0.15% carbon, up to 1.25%

manganese, up to .0.06% phosphorus, at least 0,15% 8111* fur, up to 1.0% silicon, up to 0.6% molybdenum, 12, to 14% chromium and the balance essentially iron.

3. A method according to claim 2 wherein said steel 5 is hot rolled above. 2000 References Cited UNITED STATES PATENTS I L. DEWAYNE RUTLEDGE,-Primary Examiner 15 W. W. STALLARD, Assistant Examiner 

